1. Field of the Invention
The invention relates generally to optical measurement systems. In particular, the invention relates to a method of measuring strain induced in an optical fiber during its manufacture or other processing.
2. Background Art
Optical fiber is being manufactured in large quantity for use as the transmission medium in communications networks. Numerous performance and reliability requirements imposed on the manufactured fiber can be met only by continued monitoring of the manufacturing process and its effect on the fiber. The manufacture and further processing of optical fiber involves several processes subjecting the fiber to a degree of mechanical stress and to moderately high temperatures, for example up to 125° C. The stress may be imposed by wrapping the fiber around a pulley as it is being continuously pulled through the processing station, as it is stretched between two pulleys, or as it is pulled through a die. Even if the total time that the fiber is being stressed at any point is relatively short, that time may be sufficient to induce sub-critical growth of preexisting flaws within the fiber. Such flaws are likely to impact the ultimate mechanical strength of the fiber, thereby decreasing the reliability of the field fiber. Reliability models have been created to estimate the strength of the fiber and associated fiber optic cables after their manufacture and processing. Reliability models for optical fiber are typically based on three sets of data: the size distribution of flaws or cracks with the fiber; growth parameters of fatigue cracks; and, the stress-time profile which the fiber experiences during processing. Heretofore, direct measurements of the stresses exerted on an optical fiber during high-speed processing has not been possible. As a result, the stress-time profile of the fiber has been induced from other measurements.
Furthermore, measurement of stress and strain in a manufacturing environment would be valuable for process monitoring, for equipment design, and to trouble shoot manufacturing lines.
For these reasons, it is desirable to be able to dynamically measure the stress to which an optical fiber is being subjected in a realistic manufacturing environment.